2. Learning Objectives
Tell whether a communication technology (Internet, radio, LAN, etc.) is synchronous or asynchronous; broadcast or point-to-point
Explain the roles of Internet addresses, domain names, and DNS servers in networking
Distinguish between types of protocols (TCP/IP and Ethernet)
Describe how computers are interconnected by an ISP and by a LAN
Distinguish between the Internet and the World Wide Web
Explain file structure, and how to navigate up and down the hierarchy

3. Comparing Communication Types
To understand the Internet we need to cover some basic communication vocabulary:
Synchronous Communication
Asynchronous communication
Broadcast
Communication
Multicast
Point-to-point communication

4. General Communication
Synchronous communication:
Both the sender and the receiver are active at the same time (think of talking on a telephone)
Asynchronous communication:
The sending and receiving occur at different times (think of and answering machines)

5. General Communication
Another property of communication concerns the number of receivers
Broadcast communication: single sender and many receivers (radio and TV)
Multicast: is many receivers, but usually a specific group (specialized topics)
Point-to-point communication: one specific sender and one specific receiver (telephone call)

6. Internet’s Communication Properties
The Internet supports point-to-point asynchronous communication
The Internet provides a general communication “fabric” linking all computers connected to it
Computers and the network become a single medium

7. Internet’s Communication Properties
The Internet is fast enough to mimic synchronous communication (like using a phone)
Multicasting is also possible, allowing groups to communicate in chat rooms
You can post video that can be accessed by anyone, as a form of broadcasting (compares with radio or television)

8. Internet’s Communication Properties
The Internet is a universal communication medium
The Internet also becomes more effective with each additional computer added
If x computers are already attached to the Internet, adding one more results in x potential new connections!

9. Internet Schematic Diagram

10. Client/Server Structure
Most interactions over the Internet use the client/server interaction protocol:
When you click a Web link, your computer gets the page for you...beginning the client/server interaction
Your computer is the client computer and the computer with the Web page is the server (Web server)
The client, gets services from the server
When the page is returned, the operation is completed and the client/server relationship ends

11. Basic Client/Server Interaction

12. Client/Server Structure
The client/server structure is fundamental to Internet interactions
A key aspect is that only a single service request and response are involved
The relationship is very brief relationship, lasting from the moment the request is sent to the moment the service is received

13. Many Brief Relationships
This approach means that the server can handle many clients at a time
For example, between two consecutive client requests from your browser (getting a page and asking for another) that server could have serviced hundreds of other clients
The server is busy only for as long as it takes to perform your request

14. Client/Server Relationships

15. Getting More Connected
The Internet is primarily a point-to-point asynchronous communication system
Software has been built to implement the many forms of communication
A video chat client seems to have a constant connection, but actually “slices up” the computer’s sound and video signals into into chunks, and sends each chunk separately

16. Getting More Connected
Content is transferred to the other party, whose client reassembles the sound and image for display
This process relies on fast and reliable transmission to simulate a direct connection
The Internet Protocol is generally fast and reliable enough to work

17. Appearing to Stay Connected
Even though users interact with a web site through multiple brief exchanges, many sites must give the appearance that these are part of a longer-lived connection
When using a bank web site, your login interaction must be associated with your transactions
When buying on-line, your purchases must be related to your shopping cart

18. Appearing to Stay Connected
Cookies
Server stores a small file on the client, which is returned with each request
Contains enough information to associate the interactions.
URL Parameters
Information is added to the URL
You can see this in the URL created by a Google search

19. Computer Addresses IP Addresses
Each computer connected to the Internet is given a unique address called its IP address
An IP address is a series of four numbers (one byte each) separated by dots
The range of each of these numbers (0–255) allows for billions of IP addresses
New IP addresses are in short supply

20. Computer Addresses
Each message on the Internet is called an IP packet.
Each packet is sent to a particular IP address.
Each packet may take a different route to reach that address.
A tool called Traceroute can display this route

21. Traceroute

22. TCP/IP TCP/IP Postcard Analogy
The Internet is like sending a novel to your publisher using postcards
The novel is broken into small units that fit on a postcard
The “postcards” are numbered to indicate where each belongs in the novel
As each postcard is completed, it is mailed

23. TCP/IP TCP/IP Postcard Analogy
Sooner or later, your publisher receives the postcards, but not necessarily in sequential order
Nor do they take the same route
The cards are finally arranged in order
These “postcards” are really IP packets
They hold: one unit of information, the destination IP, and their sequence number (which packet they are)

24. Packets Are Independent
Because each packet can take a different route, congestion and service interruptions do not delay transmissions
Each TCP/IP packet is independent
The TCP/IP protocol works under adverse conditions
If traffic is heavy and the packet progress is slow, the protocol allows the packet to be thrown away

25. Packets Are Independent
If a packet is killed for whatever reason, the recipient will request a resend
Packets can arrive out of order because they take different routes

26. Far and Near: WAN and LAN
The Internet is a collection of wide area networks (WAN)
These are networks that are not geographically close
The Internet is a collection of point-to-point channels
Meaning packets must visit a sequence of computers before they reach their destination

27. Far and Near: WAN and LAN
A local area network (LAN) connects computers which are geographically close
Usually they can be linked by a single cable or pair of wires
Ethernet is the main technology for local area networks
Used for connecting all the computers in a lab or building

28. Ethernet
The physical setup for an Ethernet network is a wire, wire pair, or optical fiber, called the channel
Engineers “tap” into the channel to connect a computer:
This allows it to send a signal or an electronic pulse or light flash onto the channel
All computers, including the sender, can detect the signal

29. Ethernet Party Analogy
To understand how an Ethernet network works, consider this:
A group of friends is standing around at a party telling stories.
While someone is telling a story, everyone is listening.
When the story is over, there may be a pause before the next one speaks
Then, someone typically just begins talking and the cycle starts again

30. Ethernet Party Analogy
Now, insert computer instead of friend:
A group of friends is standing around at a party telling stories.
While someone is telling a story, every computer is listening
When the story is over, there may be a pause before the next one speaks
Then, someone typically just begins talking and the cycle starts again
computers
A computer
computer
computer
a computer

31. Ethernet Party Analogy
We assumed that all “friends” were equal
No had a more import status
Everyone spoke with the same voice
There are differences, however:
Only one computer typically keeps the transmitted information
This broadcast medium is being used for point-to-point communication

32. Ethernet Party Analogy
A computer wants to transmit a message:
It starts sending signals and also starts listening to see if the message it gets is the one it sent
If it is, the computer knows it’s the only computer sending, and it completes the transmission
If it isn’t, the computer stops transmitting immediately

33. Ethernet Party Analogy
If the the transmission had to stop:
Each computer waits a random amount of time and tries to send again
Probably they will wait different amounts of time, so one will go first and the other will wait
If there is another collision, the process repeats

34. Connecting to the Internet
Today there are two basic methods:
Connection via an Internet service provider (ISP)
Connection provided by a campus or enterprise network
Most of us use both kinds of connections

35. 1. Connections by ISP Most home users connect to the Internet by ISPs
These are companies that sell connections to the Internet
The company places a modem at your house
Modems convert the bits a computer outputs into a form that is compatible with the carrier

36. 1. Connections by ISP
The signals are sent to the carrier’s business
They are converted (via modem) into a form for the server that connects to the Internet via the Internet Gateway
Digital subscriber line (DSL or ADSL) and cable (TV) are two common providers
Your smart phone also has a modem for connecting to network

38. 2. Enterprise Network Connections
The other way to connect is as a user of a larger networked organization (school, business, or governmental unit)
A LAN connects computers within the organization
The LAN is connected to the Internet by a gateway

39. Wireless Networks Variation of a LAN connection
Referred to by its protocol name
The router is:
Physically connected to an ISP’s modem
Connected to the Internet
Capable of broadcasting and receiving signals, usually radio frequency (rf ) signals

41. Computer Addresses Domain Names
It is hard to remember the numeric IP address of all the computers we communicate with
The Internet uses human-readable symbolic names for computers that are based on a hierarchy of domains
A domain is a related group of networked computers

42. Computer Addresses Domain Names Example: spiff.cs.washington.edu
The name of the computer is spiff
Which is part of the Computer Science and Engineering Department domain (cs)
Which is part of the University of Washington domain (washington)
Which is part of the educational domain (edu)

43. Computer Addresses The example shows a hierarchy of domains
Each is a member of the next larger domain
edu is a peer of other top-level domains such as com
These names are symbolic and meaningful, making them easier to read than numbers (and easier to remember)

44. The .edu Domain

45. The .edu Domain

46. DNS Servers
The Domain Name System (DNS) translates the hierarchical, human-readable names into the four-number IP address
Every Internet host knows the IP address of its nearest DNS name server
Whenever the hierarchical symbolic name is used to send information to a destination, your computer asks the DNS server to look up the corresponding IP address

47. DNS Servers
The server you ask may not know the answer. No DNS server knows all the hostnames on the Internet
Each domain has an authoritative name server, which knows the computers its domain
There are root name servers which know the authoritative server for each top-level domain
To search for a name, work through the domains down from the top

48. DNS Lookup
Your computer asks the nearest DNS server to translate a name, say
That local server asks a root name server for the IP address of the edu authoritative name server
Next it asks that edu server for the si authoritative name server
Then it asks the si server for the airandspace server, which it asks for the IP address of www
The administrator of each DNS server makes sure it has a list of root servers to get started

49. DNS Lookup Caching Redundancy
The local name server remembers the result of a lookup for a while
Skips the lookup if it already knows the answer
Redundancy
There are 13 root name servers
share load
one might crash

50. Top-Level Domains Top-level domain names (TLDs):
.edu for educational groups
.com for commercial enterprises
.org for organizations
.net for networks
.mil for the military
.gov for government agencies

51. Top-Level Domains
The top-level domains were expanded to include biz, info, name, travel, and others
The full list can be found at (ICANN is Internet Corporation for Assigned Names and Numbers)
The original top-level domains listed all apply to organizations in the United States.

52. Top-Level Domains
There is also a set of two-letter country designators (ca (Canada), uk (United Kingdom), fr (France), de (Germany, as in Deutschland), etc.)
These allow domain names to be grouped by their country of origin.

55. The World Wide Web
Some computers connected to the Internet are Web servers
Computers programmed to send files to browsers running on other computers connected to the Internet.
These Web servers and their files comprise the World Wide Web (WWW)

56. The World Wide Web Those files may be Web pages
Web servers store and send other kinds of files, too
The files are often used to:
Enhance the Web page (images or animations)
Help with other Web services (play audio or video)

57. Requesting a Web Page Web requests use client/server interaction
Requesting a Web page means your browser is a client asking for a file from a Web server
The file can be found in looking at the URL (Universal Resource Locator)
Web browsers and Web servers both “speak” the HTTP protocol

58. Requesting a Web Page The URL has three main parts:
The URL has three main parts:
Protocol: tells the computers how to fetch the file
Server computer’s name: or the server name in the domain hierarchy
Page’s pathname: tells the server which file (page) is requested and where to find it

59. Get It Right
An incorrect host name will cause DNS to find the wrong server, or return an error
An error in the path will cause the web server to get the wrong page or return an error
Administrators may arrange that a wrong URL redirects to a corrected one

60. Describing a Web Page
Servers do not store Web pages in the form seen on our screens
The pages are stored as a description of how they should appear on the screen.
The browser receives the description/source file and creates the Web page image that is described

61. Describing a Web Page
There are two advantages to storing and sending the source rather than the image itself:
A description file usually requires less information
The browser can adapt the source image to your computer more easily

63. The Internet and the Web
Some Web servers have www as part of their domain name, some don’t
Some Web servers seem to add the www if you leave it out
Some Web servers work either way (both and moma.org display the same Web site)
When is the www required and when is it optional?

64. File Structure Folders
named collection of files or other folders (or both)
also called a directory

65. File Structure Directory Hierarchy Think of any hierarchy as a tree
called the file structure of the computer and forms the directory hierarchy
Think of any hierarchy as a tree
folders are the branch points
files are the leaves

66. File Structure Directory Hierarchy
All hierarchies have branch points and leaves
Hierarchy trees are often drawn sideways or upside down
Two terms are standard, however:
Down in the hierarchy means into subfolders (towards the leaves)
Up in the hierarchy means into folders (toward the root)

68. File Structure
Part of the directory hierarchy is shown in the pathnames of this URL:
The page is specified by the pathname that tells the computer how to navigate through the directory hierarchy to the file
Each time there is a slash (/), we move into a subfolder or to the file
We go down in the hierarchy

70. Organizing the Folder
Normally, the last item in the sequence is a file name
This is not always necessary or true
When a URL ends in a slash, the last item is a folder name, and the server delivers a particular file from it, usually index.htm or index.html
The index.html file exists only if it was built

71. Organizing the Folder Why have a hierarchy?
Most people build hierarchies to organize their own thinking and work
Directories cost nothing
There is no reason not to use them
It is highly recommended

72. Summary
In this chapter we discussed the basics of networking, including the following:
Basic types of communication: point-to-point, multicast, broadcast, synchronous, and asynchronous
Networking, including IP addresses, domains, IP packets, IP protocol, WANS and LANS, Ethernet protocol, ISPs, enterprise networks, and wireless networks

73. Summary
In this chapter we discussed the basics of networking, including the following:
The difference between the Internet and the World Wide Web
File hierarchies in preparation for our further study of HTML